In the exploration for oil, it is a common technique to set casings into a borehole and introduce cement around the casing. The cement hydraulically isolates vertically separated zones to thus avoid contamination of a hydrocarbon bearing zone. It is particularly important to confirm that the cement is hydraulically secure without spaces between the casing and the cement through which hydraulic communication between vertically spaced zones may occur. Various acoustic techniques have been described to evaluate the quality of the cement bond. Such techniques may employ an ultrasonic source which is particularly effective in a casing containing water or a mixture of water and mud. In a dry casing, however, a sonic source is not practical for evaluation of the casing-cement interface.
Percussion techniques to evaluate the cement bond behind a casing have been described such as in Russian Patent Publication No. 194,694 of June 16, 1967. In U.S. Pat. No. 2,641,927 to Grable et al a percussion of the well pipe introduces vibrations which are monitored at a remote location to derive an evaluation of the cement bond. In U.S. Pat. No. 2,672,050 to Sewell a technique is described for measuring the deformability of a well casing to detect poorly bonded regions.
In an experimental technique known to the inventor hereof, a steel ball is dropped against a plate to introduce plate vibrations, which, with the measurement of the force of impact of the ball, provides an indication of the character of the cement bond behind the plate. As part of the experiment, the duration of contact of the ball with the plate and the velocity of the ball are measured. The duration of contact was found to be barely affected by the thickness of the plate or whether there exists a good or poor cement bond below the impact point on the plate. A device for measuring the rate of deceleration of a rigid body as it is impacted on material is described in U.S. Pat. No. 3,759,085 to Wilson et al.
The reliance upon the remote detection of percussion generated vibrations in a casing to evaluate the cement bond as described in the above art either yields an undesirably coarse evaluation, as in the Grable et al technique or involves complex devices which make the evaluation difficult to implement.